add `doctest_ignore_import_errors_modules`

[bsipocz]

Instead of a blank bool doctest_ignore_import_errors option, add that takes a string value for list of files/modules where imports should be ignored. E.g. that should be used to skip import issues for e.g. setup_package.py files, etc.
With this the plugins behaviour could match the pytest native doctest more, while we can keep some of the flexibility we needed for extension_helpers/astropy_helpers.

[pllim]

@astrofrog pointed out that this can be similar to https://github.com/astropy/astropy/blob/a0f5940bd00ff7259ed6b2391bfddecd007a2a43/setup.cfg#L95

He also pointed out that there is already doctest_norecursedirs but maybe that is not enough. Maybe even a new __doctest_subpackage_requires__ to prevent recursion.

[astrofrog]

I think I have a clean solution, I'll open a PR shortly

[astrofrog]

See #112